A Tourist's View of the Moon

Below is an article commissioned by Bob Guccione for the April 1971 Penthouse Magazine originally titled "A Tourist’s Guide of the Moon" by Isaac Asimov.

1969: Man reaches the Moon.

1989: Permanent colony established on the Moon.

2009: Commercial company opens shuttle crafts to Luna City. Prices reasonable. Tourists invited.

So who wants to go? What's there to see on the Moon? Miles and miles of dirty beach? The fenced off site where Neil Armstrong's footprints will be visible in perpetuity? The original American flag? Two weeks inside a dome made up to resemble an American apartment house? Is that all?

No, it isn't all! Far from all! There's a show put on by the skies of Luna that's nothing like anything ever seen here on Earth. A planetarium might put on an imitation of it, but it would be like a wax orange compared to the real thing. Just to be on the Moon and take one good look at its night-sky, even with a space helmet on, or from behind the protective glass of a Lunar observatory—will reveal at once that what we call a night-sky on Earth is nothing but a pale washed out substitute.

There are no clouds on the Moon, no mists, no fog, no city lights; nothing to hide the stars, nothing to dim them, nothing to drown them out. There is no atmosphere to absorb starlight, so that each star shines nearly half a magnitude brighter than it does on Earth, and keeps its brightness from zenith all the way to horizon. This means that many stars, just under the threshold of visibility seen from Earth, are bright enough to be clearly visible in the skies of the Moon. The unaided 20/20 eye on Earth, under the best conditions, can make out perhaps 2,500 stars in the sky at any one time. On the Moon, the same eye would make out nearly 6,000 stars. These hordes of dim stars, not seen on Earth by the naked eye, would end the familiar constellations a richness undreamed of here.

What’s more, the stars of the Lunar sky wouldn't twinkle. It is the temperature differences in air that bend the tiny starlight-beams this way and that, making the little sparks constantly shift position. This gives the stars a soft beauty but it wearies the eye. On the airless Moon, each star, however faint it might be, is fixed. The eye can follow the patterns of the constellations; The mind can create associations and pictures in those patterns in endless variations.

And there will be ample time to study those patterns too, for the Lunar night lasts 348 hours—or just over two weeks.

Sun from the Moon

Then the Sun rises in the east. It's the same Sun we see from Earth and appears to be the same size—but it's a lot more dangerous. There's no air to soften the radiation blow and to absorb the X-rays and far ultraviolet rays long before they reach us. There's no ocean to absorb the heat and keep the temperature rise moderate. There are no winds and currents to spread the heat. At the Lunar equator, the temperature at noon reaches that of the boiling point of water.

But that's on the surface, of course. In Luna City, underground, the temperatures will always be moderate. And through television cameras one can still watch the sky. On the Moon, where there is no air to scatter the light, the daytime sky remains black; utterly black. The Sun's light reflected brightly from the Moon's crunchy surface would dazzle the eye and limit the clarity with which one could see the stars. But suppose you watch from inside the dome, with the television cameras turned to a section of the sky. The brilliant ground would be invisible and if the Sun itself were not in the field of vision, the stars would be as visible by day as by night.

The Sun itself would be a rare sight to behold—though not directly, of course. It is easy to imagine an opaque region on the television screen just large enough to cover the shining disc of the Sun. Suppose this opaque region were an exact fit and were shifted by a timing mechanism so that it continued to stay in front of the Sun as that body moved slowly across the heavens.

On Earth, this would make no difference. The atmosphere would remain full of light; the sky would remain blue; the stars would remain invisible. On the Moon, however, with no air to scatter light, hiding the disc of the Sun would give the effect of having no Sun at all in the sky—except that if the opaque region were an exact fit, the bright red rim of its atmosphere would be visible. The corona would be seen. The Sun would be in total eclipse for as long as the opaque cover is maintained, with all the beauty that can be seen only so rarely on Earth.

Earth from the Moon

The Moon's sky offers us something we couldn't see at all from the Earth under any conditions—something so remarkable it is worth traveling the distance for. In the Moon's sky, there is the Earth! The Earth is, so to speak, the Moon's "Moon." But what a difference! The Earth is nearly four times as wide as the Moon, and so it appears, in the Moon's sky, four times as wide as the Moon does in ours.

The brightness of an object in the sky depends, not on its width but, all things being equal, on its area, which is the square of the width. On that basis, the Moon's Earth ought to be 14 times as bright as the Earth's Moon. But all things are not equal. The Moon's visible surface is bare rock that absorbs most of the sunlight that falls on it. Only about 7 percent of the sunlight falling on the Lunar surface is reflected back into space. The Earth, on the other hand, has an atmosphere more or less filled with clouds, and these are much better mirrors than bare rock is. Some 28 percent of the sunlight that falls on the Earth's atmosphere is reflected back into space. The Earth, square mile for square mile, reflects four times as much sunlight as the Moon does. Combine this with the Earth's greater visible area and the Earth would be 56 times as bright as the Moon.

Remember, though, that there is no atmosphere on the Moon to absorb the Earthlight. Adding 30 percent for that reason allows us to end with the fact that the Earth, as seen from the Moon, is just about 80 times as bright as the Moon is seen from Earth. If the brightness of the stars, as seen from the Moon, offers a romantic and beautiful sight, what are we to say of the large "Moon" presented by Earth, and the brilliance of Earthlight?

The Earth, as seen from the Moon, passes through phases, just as the Moon does, as seen from the Earth—and in the same period of time. The Earth phases are exactly opposite to those of the Moon, however. When it is the time of the new Moon on Earth, it is the time of the full Earth on the Moon and vice versa. At full Earth on the Moon, with the Earthlight at its maximum (80 times as bright as the brightest full Moon any Earthman has ever seen from the surface of our planet), the Lunar landscape is lit by light without heat, throwing the surface into soft highlights surrounded by black shadows—like Sunlight but without any of its harsh and dangerous effects and with a glow nothing on Earth can duplicate.

Nor is this something we need just imagine; We can see the Earthlight on the Moon when the Moon is new. Then Earth is full (as seen from the Moon). When the new Moon's crescent is thick enough to linger in the sky for an hour or so after sunset, so that the sky is dark, we can see beyond the pale crescent the faint outlines of the rest of the Moon lit by Earthlight.

Since the Moon always presents nearly the same face to the Earth as it circles us, the Earth seems nearly motionless in the Moon's sky. If we were standing on some point on the Moon near the center of its face (as seen from the Earth), the Earth would appear directly overhead and would more or less stay there. If we were to stand north of the central point on the Moon's face, the Earth would appear south of the zenith. The farther north we were standing, the farther south the Earth would appear. If we were standing east of the central point, the Earth would appear west, and so on.

But wherever the Earth appeared, there it would stay, and through the month we could watch its phases change from new to full and back again. Nor is it only the slow phase-change we could watch. The Earth's face is far more variegated than the Moon's is. The Moon presents us only one face forever and that smooth expanse of light is unbroken by water, untroubled by air. Not so the Earth's face, which has ever-shifting clouds forming their curling faint-blue patterns. And through the clouds, one can glimpse the deeper blue of ocean, the faint tawniness of desert, the touch of mild green that is the evidence of life. Occasionally, the outline of a continent might be made out. Those parts of the outline most often seen would be the desert areas where clouds are few—the bulge of African Sahara, the polygon of Arabia, the curve of Australia or the Chilean coastline, the thin extent of lower California.

Rotation on the Moon

The Earth rotates, too, once in 24 hours, so that each part is presented to the eyes of the Moon-tourist in turn. Because the variations are endless, the interest can never fail.

Additional interest arises out of the fact that the Earth does not hang quite motionlessly in the sky, because the Moon's orbit about the Earth is not an exact circle. The Moon moves in an ellipse, and moves more slowly at some parts of its orbit than others. Without going into detail to explain why, this uneven speed results in the Moon not presenting quite the same face to us at all times. During part of its orbit, it turns a little so we can see just a small way beyond its eastern edge, and during the rest of its orbit it slowly swings back so that we can see just a small way beyond its western edge. This is called the Moon's "libration."

The effect to someone standing on the surface of the Moon is to make the Earth swing back and forth over a one month period about its average position in the sky. In some conditions it would shift as much as 16 degrees this way or that. This means that if its average position were at the zenith, it could shift one-sixth of the way toward the horizon before swinging back. This shift would not be very spectacular if the Moon were high in the sky, but suppose it were low in the sky. Suppose a tourist on the Moon were standing near the eastern (or western) edge of the face of the Moon turned towards us. If we imagine ourselves watching this tourist through a telescope, we would see the Moon's libration carry him beyond the visible edge and then back again, over and over. What the tourist on the Moon would see would be the huge globe of the Earth sinking toward the horizon, then vanishing below it, and then rising above it eventually, only to begin sinking again—over and over. There remains one spectacle involving the Earth that would be a must for any tourist. This involves the combination of Earth and Sun. The Sun, as seen from the Moon, moves across the sky more slowly than when seen from the Earth, for the Moon rotates about its axis only once in 29 1/2 Earthdays. The Sun rises, spends 14 days crossing the sky, then sets, and spends 14 more days making its way back to the point of Sunrise again. What would happen when Sun and Earth were in the same part of the sky?

The path followed by the Sun in the Moon's sky is such that ordinarily it passes either above or below the Earth. The Earth's narrow crescent shifts position from east to west, around the northern or southern edge of the Earth. The amount by which the Sun misses the Earth's disc as it crosses from east to west varies. Every once in a while, in the process of passing first on this side, then that, the Sun manages to make a direct hit, so to speak, and passes behind the Earth's disc. When that happens, Sunlight cannot fall on the Moon, and what we see from Earth's surface is a Lunar eclipse. The bright face of the full Moon (a Lunar eclipse always takes place at full Moon) is bitten into by Earth's shadow. If the Sun passes behind the Earth well away from the edge of Earth's disc, the entire face of the full Moon is hidden.

Eclipses on the Moon

How does this appear as seen from the Moon's surface? A Lunar eclipse, seen from Earth's surface, is a Solar eclipse seen from the Moon's surface. But the Moon's version of a Solar eclipse is different from ours in two ways. It is slow-motion—it takes as much as one full hour for the Sun to pass entirely behind the Earth and it can take up to nearly three hours before it begins to appear again at the Earth's other edge. (Compare a three-hour Solar eclipse on the Moon with one that lasts for an absolute maximum of seven minutes as seen from the Earth.) Also, the Earth's disc is so huge that it covers not only the Sun itself but much of its corona. So the corona of the Sun is never as spectacular a sight during the Solar eclipse on the Moon as it is during the Solar eclipse on the Earth.

There is something else, though, that more than makes up for this. The Earth has an atmosphere, which the Moon hasn't. When the Sun is behind the Earth's disc, its light shines through the atmosphere all around the Earth. Most of that light is absorbed or scattered by the atmosphere but the longest lightwaves survive. This means that the invisible black circle of the Earth's disc is surrounded by a rim of bright orange and what we see is, in effect, a curve of Sunset all around the Earth.

Picture, then, the Solar eclipse as seen from the Moon. The black sky is covered with a powdering of stars much more thickly than here on Earth, and somewhere in that sky is a perfect circle of orange light beyond which is what can be seen of the pearly white of the Sun's outer corona. And the surface of the Moon itself is lit for a while not by the harsh and brilliant white light of the Sun, nor by the cool and soft white reflected light of the Earth, but by the dim and orange light of another world's Sunset.

Is this just imagination? Not at all. We can actually see that Sunset light from the Earth, for during a total eclipse of the Moon, we generally don't see the Moon disappear. It remains visible, shining with a dim copper color in the distant Sunset-glow.

The Solar eclipse by Earth is the supreme sight of the Lunar skies. That is what the tourists will wait for confidently, since the moment of such eclipses can be predicted centuries ahead of time.

Some things cannot be predicted, however. It can be that at the moment of the eclipse, those sections of the atmosphere rimming the Earth are unusually full of clouds so that little light will get through. The orange circle will be dim, or incomplete, or even virtually absent, and the tourists will be disappointed. (On certain rare occasions, the fully-eclipsed Moon does just about disappear, and we know the distant Sunset circle has failed.) Will there be "eclipse insurance" taken out by tourists travelling to the Moon, to guard against total loss of passage fare in case of this happening?

Nostalgia on the Moon

What is left in Earth's sky the Moon cannot match? A shooting star, perhaps? Many meteors hit the Moon, but they must pass through atmosphere if they are to glow. The beauties of our Sunrise and Sunset depend on the presence of an atmosphere and the same phenomena on the Moon are dull and colorless in comparison. Then there are the ever-changing cloud patterns in the sky; the mist, the fog, the rain, the snow. None of this ever appears on the Moon.

There is even the sight of the calm, deep, unbroken blue of the sky of a peaceful summer day, when a person can find himself in open air stretching for endless miles in all directions and with no need for any protective garment or any curving dome to protect him against the environment. We have all about us the infinite disregarded wonder of the Earth.

Recommended Reading

Best known for his novels and short stories, Isaac Asimov also wrote many essays that showcased his intellect and his visions of the future. Explore more of these essays in The Roving Mind.

The Roving Mind is a collection of wide-ranging essays, reflecting Asimov's skill for disseminating knowledge. The 62 essays cover topics such as creationism, censorship, population, transportation, philosophy of science, computers and corporations of the future, pseudoscience, and astronomy, as well as several personal stories from his life.